Cancer (malignant tumor) is the most frequent cause of death in Japan, and one out of two Japanese is said to suffer from the disease. In addition, cancer is one of the leading causes of death among other developed countries. Accordingly, development of an effective treatment method for cancer has been a long-lasting goal of most people in the world including Japan. Cancer, however, derives from cells of a patient himself/herself. Thus, an effective therapeutic agent and/or method capable of effectively treating cancer without an adverse effect have not been easily developed, so that a potent therapeutic agent or method has yet to be reported.
Examples of current major cancer treatment include surgical treatment, chemotherapy, and radiotherapy. Recently, the treatment has extended to include thermotherapy and photodynamic therapy.
The above thermotherapy is a treatment method which takes advantage of the fact that cancer cells are heat-sensitive in nature compared to normal cells, and specifically inhibits proliferation of the cancer cells. That is, when a tissue containing cancer and normal cells is heated at about 42 to 43° C., homeostasis plays a role in the section of the normal cells to expand blood vessels surrounding the cells, thereby increasing their blood flow and dissipating heat. This allows the temperature of the cells to be kept at about 40° C., which results in almost no damage. In contrast, the section consisting of the cancer cells generally has insufficient blood flow, etc., and their homeostasis thus does not adequately function. As a result, the temperature of the cells rises to at or near the heating temperature, and the cells are killed at about 42.5° C. In this regard, however, when there is a large heating area in thermotherapy or when cancer to be treated is at some depth from the body surface, the patient's burden due to heating unfortunately increases.
The above-described photodynamic therapy is a treatment method including: administrating a photosensitizer or a precursor thereof having an affinity for a cancerous tissue to a patient; subsequently irradiating the cancerous tissue with a laser beam to excite the photosensitizer; and generating reactive oxygen to kill cancer cells. For example, Patent Document 1 discloses a photodynamic therapy formulation in which at least one drug, including a photosensitizing compound, is encapsulated inside a lipid membrane or in an internal aqueous phase thereof and in which a liposome substantially free of an organic solvent is included. Patent Document 1 also discloses that protoporphyrin IX (PpIX) is among preferable photosensitizing compounds, and that 5-aminolevulinic acid (5-ALA), which is a PpIX precursor, selectively induces PpIX production in tumors. In addition, Patent Document 1 describes a combination between photodynamic therapy and thermotherapy.
In view of the above, 5-ALA is a PpIX precursor which is a photosensitizer. Accordingly, when 5-ALA is used for cancer treatment, at least photodynamic therapy is implemented, and occasional use of a combination with additional thermotherapy has been previously known. So far, no cancer treatment applications using 5-ALA but not using the photodynamic therapy have been reported.